1. Field of the Invention
The present invention relates to an arbiter for the local bus of a computer network.
2. Description of Related Art
Computers are sometimes coupled together within a network such as a local area network (LAN), a wide area network (WAN) and/or a metropolitan area network (MAN). Networks allow computer systems to communicate with each other and improve the utility of the computer.
FIG. 1 shows one type of computer network. The network contains a pair of remote local busses coupled together by a communication link and a pair of bridges. The local busses contain various agent devices such as memory and a microprocessor. The agent of one local bus may request access to an agent located on the other local bus. For example, the microprocessor of one local system may request access to memory space that resides within a memory device located in a remote system.
There have been developed various standards and protocols for network systems, including a standard for remotely accessing memory commonly referred to as the Non-uniform memory access(NUMA) system. NUMA systems typically employ destination queuing (commonly referred to as DEFER) or source queuing (NACK). To access a remote memory device in a NUMA system, the requesting agent generates a memory access request that is transmitted to the bridge on the local bus. The memory access request is retransmitted by the bridge to the remote system. If the requesting agent supports DEFER, the bridge will send a DEFER response to the agent which releases the local bus for further use. Meanwhile the remote bridge performs the requested transaction on the targeted local bus. The requested transaction is relayed back to the originating system through the bridges. A DEFER REPLY is provided to the requesting agent to complete the transaction.
Not all agents are NUMA or DEFER compatible. Placing a DEFERless agent within a NUMA system may reduce the efficiency of the agents within the local system. For example, when a DEFERless agents generates a memory access request and the bridge is unable to immediately service the request, the bridge typically responds with a negative acknowledge (NACK). The DEFERless agent then resubmits another access request. The cycle of access requests and NACK responses may continue for a significant amount of time, thereby tying up access to the local bus and hindering operation of the remaining local agents. It would be desirable to provide a NUMA based computer network which can efficiently accommodate a non-NUMA agent.